DEN83LU005

After takeoff, the plt of the weight-shift controlled, ultralight vehicle entered a steep climb, then began a series of maneuvers which included past vertical wingovers. Reportedly, on his last maneuver, he performed a whip stall. A ground witness stated that during the recovery, he 'pulled in tight to get an impressive dive.' A plt witness & 2 FAA inspectors reported that when the nose dropped, it went past a vertical position. The plt witness stated that during the recovery, the plt held the nose in aprx a 60 deg dive for 3 to 4 sec. The FAA inspectors reported that during the pull-out, the wings failed. An exam revealed that the left & right inboard compression struts had buckled from positive loading. The plt had modified the ultralight by installing a non-standard eng-prop reduction drive sys, by changing the pitch trim, by adding a lower tail boom brace & by adding a 10 lb wt to the nose to increase the forward speed in a dive.